Vibration fatigue testing method and machine



SR xR 2,450,939

Oct. 12, 1948. P. COR

VIBRATION FATIQUE TESTING METHOD AND MACHINE Filed April 20, 1940 2 Sheets-Sheet'l INVENTOR.

fvrre Cor.

2 Sheets-Sheet 2 INVE/V TUR.

Pierre C02. 1 /MM fa 2% Patented Oct. 12,1948

UNITED OFFICE VIBRATION FATIGUE TESTING METHOD AND MACHINE Pierre Cor, Paris, France Application April 20, 1940, Serial No. 330,782 In France April 20, 1939 Section 3, Public Law 690, August 8, 1946 Patent expires April 20, 1959 6 Claims.

The present invention relates to methods and machines for testing vibration fatigue of materials subjected to alternate bending and its object is to provide a method and machine of this kind which is better adapted to meet the requirements of practice than those used for the same purposes up to this time.

A preferred embodiment of my invention will be hereinafter described with reference to the accompanying drawings, given merely by way of example, and in which;

Fig. 1 is an elevational view of the machine.

Fig. 2 is a corresponding plan view.

Fig. 3 is a diagrammatic view of an indicating device.

Referring to the drawings, the machine essentially comprises a main frame I on which are mounted on a fixed vise 2, and a movable vise 3 which is subject to longitudinal movement in order to change its distance from the fixed vise 2. The vise 3 is guided upon fixed rods 4 which are mounted at one end upon the fixed vise 2 and at the other end upon a fixed angle piece 5. The movement of the vise 3 is effected by an endless screw 6. The test piece I, which consists, for instance, of a strip of the material to be tested, is permanently held at one end by the vise 2. It is engaged at its other end, by easy friction, in the movable vise 3, and it projects beyond this vise for a certain length l which may be termed length of synchronism, for reasons which will be further stated.

The hand-wheel 3 for clamping the movable vise is advantageously provided with a conventional device for limiting the clamp torque.

At its free end. the said test piece is subjected to the action of an electro-magnet which is,

designated in a general way by B and is supplied with alternating current. The said electro-magnet gLves to the test piece a movement of oscillation in a direction perpendicuar to its plane, as shown by the dotted lines in Fig. 2, and the frequency of this oscillation is equal to the frequency of the exciting current, or to a multiple of this frequency. The attraction of the electro-magnet upon the strip will take place by the variation of the magnetic field. If the test piece should consist of non-magnetic material, its free end will be provided with a mass of magnetic material 9. Preferably, the exciting current of the electromagnet has but a small value, in order that the machine may be connected with a lighting circuit.

The operation of the machine is as follows:

After the test-piece 1 has been put in place 2 and held at one end in the fixed vise 2, the movable vise 3 is slightly clamped, in such way that it will be free to move along the test piece. The

'electro-magnet 8 is connected in the circuit, and

it is regulated for a weak field or is placed at a considerable distance, and the operator seeks for a position of the movable vise which will afford a. great amplitude. It is observed that for certain positions, the amplitude will pass through a maximum. At this time, the strip will vibrate in synchronism between its natural frequency of oscillation and the frequency of the alternating current. There are several distinct positions of synchronism which correspond, for instance in the case of excitation at the frequency of 6000 per minute (which corresponds to an alternating current of 50 cycles per second), to vibrations of the test piece having frequencies of 1500, 3000, 6000 or 12000.

It is shown by experience that in the present case, it is at the frequency of oscillation of 6000 that the amplitude of oscillation of the piece is a maximum. It is an easy matter, by means of the stroboscope, to find this frequency of oscillation.

The free length l of the piece, which is the, length for which the piece will vibrate in synchronism with the frequency of the current of oscillation, will in fact constitute what may be termed the length of synchronism.

This length of synchronism provides for the measurement of the modulus of elasticity of the material under test, or of checking it in a few moments. This is a particular advantage of the present invention.

After this adjustment of the position of the movable vise 3, the operator can obtain, by moving the electro-magnet 8, amplitudes of oscillations of such great value that any kind of material will break in a few minutes. He will thus seek to adjust for an amplitude of such value as will afford a beginning of breakage within a reasonable time. This adjustment of the position of the electro-magnet is possible, as its support if! can be moved upon guides II and I3, by screws l2 and M or the like.

When the test piece begins to crack in a visible manner, even to a very slight degree, its amplitude will increase, and thus any commencement of cracking is shown at once. Moreover, as the amplitude has now increased, this cracking will increase at a greater rate.

As the synchronism is only exact as regards its maximum, it is thus maintained for some time, and hence the amplitude can continu to increase.

Then at a certain time, the vibration will suddenly cease, or at least it will abruptly assume a very small value. In this case, by the use of a device for properly measuring the amplitudes (for instance by spring-mounted keys or by an optical method), it can be rapidly ascertained whether the strip will break or not. An instrument for the recording of amplitudes can be readily added to this device. It has been observed, during numerous tests, that a crack whose origin is due to a scratch or to an imperceptible defect, will at once increase if the limit of strain is exceeded even to a very slight degree and this increase will take place according to a formula which is slower at the start according as the stress upon the material exceeds the elastic limit to a slighter degree, after which the rate continues to increase.

Owing to the invention, it is thus possible, with the use of very simple means for measuring the amplitude, to make tests which will not last for more than two hours.

As to the relations between the various quantities relating to the test piece, such as thickness. length, density, modulus of elasticity, radius of curvature and limit of strain, these are found by calculations which permit, in conformity with experience, of preparin simple calculating tables which will at once show the desired results.

Even without using such means, it is possible to establish series of comparative measurements, in which the value of the stress is proportional to the expression:

Mod. of elast.XthicknessXamplitnde Square of the length i In the present case, it is supposed that the test piece is attracted by a fixed electro-magnet which is excited by an alternating current. A like result may be obtained by the use of permanent magnets which are rotated by a synchronous motor. Moreover, it is evident that the several parts of the machine can be modified to any desired extent.

The principal advantages of the invention can be summarized as follows:

1. It provides for direct tests upon sheet metal pieces in their condition of delivery, including the drawn state, and whatever be the condition of their surface.

2. As the test will last for about two hours, the machine can be used for the ofiicial approval of delivered goods, and the test for strain can be made a standard test.

3. As an accessory feature, it provides for a very rapid measurement of the modulus of elasticity.

Moreover, the said machine will provide for the following tests:

(a) The alternate bending of sheets or plates, by the use of an ordinary test piece having an aperture for the mass, for non-magnetic material.

(27) The alternate shearing of assembling systems. such as bolts, rivets, spot welding, welding by milled wheels, or autogeneous welding. It is simply necessary to make the same test with a pair of test pieces which are joined at the vibrating end by the assembling system to be tested. lhe slipping between the two test pieces which Is produced by the bending. will cause the shearing stress. This will afford absolute results, the only condition being the measurement of the modulus of elasticity of the test pieces in use, and this is readily eifected.

(c) The longitudinal shearing of the welded joints. This same device with double test-piece is employed, with welding along the two edges of the vibrating end.

(d) Contraction and compression upon any material or welded joint. It is simply necessary, with the same double test piece, to insert the part under test into one of the test pieces which has been cut for this purpose. The part should be inserted near the vibratin end (practically no curvature).

(e) Alternate bending, without the use of clamping, this being applicable to all specimens, and especially to welded pieces. The operation is commenced by finding the length of synchronism of, the sheet metal piece, then taking a test piece which has a part whose width exceeds the clamped part. This extra part is connected with the vibrating part by a suitable portion which reduces the effect of the notching to a very small value.

(I) Alternate bending of round bars or tubes by means of jaws of suitable form. The masses to be employed will be determined by experience.

(g) Alternate torsion. It is simply necessary to use a small hinge device, in which the axle pin consists of the material under test. One part of the device is clamped in the movable jaw, and the other part is provided with a rod ending in an iron armature which is pivoted between the poles of the electro-magnet. The synchronism is adjusted by a mass which is slidable on the said rod. Each part of the hinge is pressed upon the common axle pin, which is now subjected to torsion.

It is obvious that this enumeration of the principal forms of possible tests is given solely by way of indication, and is not of a limitative nature.

In some cases, it may be advantageous to add to the alternate force acting upon the test piece, a force of constant value, such as the action of a pole of a permanent magnet which is situated at an adjustable distance.

The tests for strain can be made in another manner. Instead of adjusting the eifective length of the test piece in such way that its natural frequency of oscillation is in synchronism with the frequency of the exciting magnetic field or with a multiple of this frequency, this effective length can be made such that it will be somewhat short as compared with the theoretical length for the absolute synchronism. In these conditions, any decrease in the operating cross-section of the test piece will bring this piece near the conditions of maximum synchronism, as this reduction of the elasticity will have the efiect of lengthening the period. The curve for the amplitudes of a test piece relatively to its length, when under the influence of a constant periodic action, will show maxima in the form of very sharp peaks which correspond to the given frequency and its harmonics. The increase of the amplitude caused by a better synchronism due to a crack, will thus be considerable as compared with the importance of the crack.

As this increase of amplitude will cause an additional increase of the crack, it is evident that the present system is of a most sensitive nature, and.

it thus affords very exact indications in a very short time.

It should be further remarked that the length of synchronism is adjusted before the movable vise is entirely clamped. However, the positive clamping will shorten the test piece, and thus the condition for the short adjustment is usually obtained in an automatic manner.

The present invention further relates to an indicating device cooperating with the said machine, and providing for the measurement of amplitudes and for the automatic warning of any commencement of breakage. A constructional form of this device, which is chiefly employed for tests in which the adjustment is made above synchronism, is shown diagrammatically in Fig. 3.

It should be stated in the first place that a similar device is provided on each side of the test piece, each of which measures half the amplitude.

Each of the said devices comprises, as shown in the drawings, a fine wire i5 (such as a brass wire of 0.5 mm. diameter) which forms a very light spring.

The said wire I5 is secured by a. screw Hi to a support I! upon which the said wire is mounted at 18 in a very exact position. The said support is provided at its lower part with a plate carrying an indicating mark l9, and adapted to slide in a position adjacent a graduated ruler 2| which forms a supporting and a guiding device, and which is slidable on its support which is secured to the table of the machine.

The sliding movements of the support I! relatively to the ruler 2|, and of the device ll-2l relatively to the table of the machine, are effected, for instance, by means of the milled knobs 22 and 23.

The automatic signalling is effected by an electric circuit which is supplied at low voltage by'a small transformer 24, in which circuit is mounted a lamp (for instance a pocket lamp). When the wire [5 is mounted at 18 on the support H, the electric circuit is closed, and the lamp lights up, but when the test piece comes upon the wire l5, the circuit is broken, and the lamp 25 is put out.

The adjustment is made as follows.

1. The operator, by pressing the button 22, brings the mark 19 of the plate 20 next the zero point of the ruler 2|.

2. He presses the button 23, and thus brings the device consisting of the ruler 2| and the support I! into such position that the wire 15 will make a close contact with the test piece.

3. By pressing the button 22, the wire is is removed from the field of motion of the test piece.

4. The test piece is set in vibration, and the operator presses the button 22 in order to obtain the exact contact between the test piece and the wire l5. The parts are thus separated by a very short distance.

After this has taken place, the least increase of the amplitude of the strip will cut out the lamp 25.

What I claim is:

1. A machine for testing the vibration fatigue of materials in the form of test pieces. comprising a main frame, a fixed vise mounted upon said main frame and adapted to clamp one end of said test piece, a vise whose position is adjustable along the test piece, a clamping device mounted on the said adjustable vise and adapted to clamp the test piece at an intermediate point with a given pressure, a first carriage slidable on the said main frame, a second carriage slidable on the first carriage in a direction perpendicular to that of the first carriage, an elec'tro-magnct mounted on the second carriage and adapted to act upon the free end of the test piece in order that it will oscillate,.and indicating means which are responsive to the amplitude of the oscillations of the test piece.

2. A machine for testing the vibration fatigue of materials in the form of test pieces, comprising a main frame, a fixed vise mounted upon said main frame and adapted to clamp one end of said test piece, a vise whose position is adjustable along the test piece, a clamping device mounted on the said adjustable vise and adapted to clamp the test piece at an intermediate point with a given pressure, a magnetic mass mounted on the free end of the test piece, a first carriage slidable on the said main frame, a second carriage slidable on the first carriage in a direction perpendicular to that of the first carriage, an electro-magnet mounted on the second carriage and adapted to act upon the said mass, and indicating means which are responsive to the amplitude of the oscillations of the test piece.

3. A machine for testing the vibration fatigue of materials in the form of test pieces, comprising a main frame, a fixed v se mounted on said main frame and adapted to clamp one end of said test piece, a vise Whose position is adjustable along the test piece, a clamping device mounted on the said adjustable vise and adapted to clamp the test piece at an intermediate point with a given pressure, a first carriage slidable on the said main frame, a second carriage slidable on the first carriage in a direction perpendicular to that of the first carriage, an electro-magnet mounted on the second carriage and adapted to act upon the free end of the test piece in order that it will oscillate, a support whose position is adjustable on the said main frame, elastic means mounted upon said support and adapted to be actuated by the free end of the test piece, and indicating means adapted to be actuated by the said elastic means.

4. A machine for testing the vibration fatigue 2 of materials in the form of test pieces, comprising a main frame, a fixed vise mounted on said main frame and adapted to clamp one end of said test piece, a vise whose position is adjustable along the test piece, a clamping device mounted on said adjustable vise and adapted to clamp the test piece at an intermediate point with a given pressure, a first carriage slidable on the said main frame, a second carriage slidable on the first carriage in a direction perpendicular to that of the first carriage, an electro-magnet mounted on the second carriage and adapted to act upon the free end of the test piece in order that it will oscillate, a support whose position is adjustable on the said main frame, an elastic metal wire mounted on the said support and adapted to be actuated by the free end of the test piece, an electric contact adapted to be actuated by the said Wire, and an electric signaling device actuated by the said contact.

5. A method of testing the vibration fatigue of a test piece under alternating stress which comprises supporting the test piece so that a portion thereof can vibrate freely, applying a periodic excitation of known frequency to said portion of the test piece, adjusting a dimension of said portion so that the natural frequency thereof differs slightly from the exciting frequency, whereby the change of natural frequency consequent upon the development of an incipient change of state of the material will bring about resonance with the excitation, and observing changes of amplitude of the oscillation of said portion of the test piece.

6. A machine for testing the vibration fatigue of an elongated test piece which comprises, in combination a frame, two means for clamping said test piece with respect to said frame located 2,450,939 7 8 at a distance from each other, one of said clamp- REFERENCES CITED ing means being adapted to cooperate with one end of said piece and being fixed in position on g g g g ggg s are of record m the said frame, the other clamping means being adjustable in position on said frame to cooperate UNITED STATES PATENTS with any desired point of an intermediate portion Number Name Date of the length of said piece, and exciting means for 1,635,787 Hort July 12 1927 periodically urging the portion of said test piece 2 156 592 Knox May 1939 outside of said clamping means at a given free- 21713o3 DeForest Aug 1939 quency in a direction perpendicular to its axis. 10 2429094 K t I''''" To PIERRE COR. ent e a Oct. 14, 1947 

